The present invention relates to an optical device; and, more particularly, to a method for fabricating an optical device integrated with a spot size converter capable of coupling an optical device with optical fibers without any lenses in a transmission module for an optical fiber communication.
Generally, after manufacturing of an optical device, such as a semiconductor layer diode (LD) or a semiconductor optical amplifier (SOA), optical fibers are coupled at ends of the optical devices, thereby being formed into a type of a module. In this case, to obtain high coupling efficiency, complicated optical instruments should be used, which leads to high production cost and complex production process.
In particular, to use a laser diode (LD) for communication as a common optical device, it is necessary to bring down the cost of a transmission module. Therefore, a method coupling a complicated optical instrument such as a lens with optical fibers is not efficient because it raises the production cost. Besides, it has a difficulty in controlling the position of the lens.
Also, when the both sides of an active layer have a predetermined reflectivity, an optical ray comes to feedback and as a result of it, the performance of the semiconductor optical amplifier (SOA), which has been in the spotlight as an alternative to a conventional Erbium doped fiber amplifier (EDFA), is deteriorated.
Accordingly, when a laser diode or a semiconductor optical amplifier is fabricated and integrated into a module, a spot size converter (SSC) made of a tapered waveguide through selective growth is integrated at the front end of the laser diode, or at both ends of the semiconductor optical amplifier to improve the coupling efficiency with optical fibers.
A conventional semiconductor waveguide is very small in size and has a big difference from a clad layer in the refraction index, so the spot size is relatively small compared to optical fibers. This small spot size is given a large radiation angle when it comes out, and such that the coupling loss with optical fiber becomes very large.
To solve this problem, a tapered region is formed in a predetermined area of a device and far-field angle is reduced by converting spot size. This way, it becomes possible to connect optical fibers highly efficiently without any extra optical instruments. Additionally, it enhances the effect of a spot size converter further lowering the reflectivity by a tilted waveguide.
In fabricating the spot size converter, the core technology is to connect the optical device with optical fibers by narrowing the radiation angle of a ray coming out of the optical device, and for this, a tapered waveguide is required.
The tapered waveguide can be embodied by controlling its thickness of the waveguide over 3:1 through a selective regrowth process. The selective regrowth process inevitably forms a butt-joint portion BJ. Especially, in case of the SOA, the amplifier cannot perform its role as an amplifier when a reflection is made in the butt-joint portion. Therefore, the antireflecting connection should be performed necessarily.
Recently coming up as important technologies are a method that overcomes the strain caused by the selective growth which has thickness difference of over 3:1 in case of applying an SSC to an optical device and forms a quality optical waveguide, and a method minimizing reflectivity in the butt-joint portion.
In other words, the coupling efficiency with optical fibers should be heightened with a radiation angle of more or less 10xc2x0, and the reflectivity should be made very low in the connection region, i.e., butt-joint portion. Particularly, a junction plane should be formed tilted to reduce the reflectivity, but there is a problem that the regrowth is performed on a surface whose crystal plane is not exposed clearly.
Consequently, with this method, an undesired defect is caused, and because the quality of the layer makes a remarkable drop, it is hard to achieve the goal of lowering reflectivity. Moreover, the process is not reproducible.
FIG. 1A is a plane figure showing a laser diode with a vertically tapered spot size converter in accordance with a conventional method, and FIG. 1B is a cross-sectional view of FIG. 1A cut along the line I-Ixe2x80x2.
Referring to FIGS. 1A and 1B, after growing an n-InP clad layer 12 on an n-InP substrate 11, an active layer 13 of an InGaAsP/InGaAsP multi quantum well structure is grown on the n-InP clad layer 12, and on the active layer 13, a p-InP clad layer 14 is grown. Subsequently, a photolithography etching is performed to define the width of the active layer 13 around 1 xcexcm . Then, a mask 15 for selective growth is formed on the etched active layer through a photolithography etching method, and dry- or wet-etching is carried out with the mask 15 for selective growth so as to expose the (011) plane or (0-1-1) plane of the active layer.
Subsequently, a vertically tapered spot size conversion region 16 is formed by using a selective growth method according to the migration of a material gas injected into the mask 15 for selective growth in a metal organic chemical vapor deposition (MOCVD). In the conventional method, the mask 15 for the selective growth that can control the growth difference over 3:1 to have the spot size conversion region 16 vertically tapered. However, as a defect is caused in the butt-joint portion 17 by the vertical tapering, the light coming from the active layer 14 is not transmitted to the spot size conversion region 16 and partly reflected. Due to the reflection in the butt-joint portion 17, the light goes back to the optical device and oscillates a laser, thus canceling the function of the amplifier.
To suppress the reflection in the butt-joint portion and solve the problem, a method was suggested that the butt-joint portion is made tilted to change the direction of the reflection light different from a resonator not to oscillate a laser.
FIG. 1C illustrates the butt-joint portion BJ, the junction plane of the active layer of a multi quantum well (MQW) structure and the spot size conversion (SSC) region, being vertically tapered with a plane slanted towards the vertical at a predetermined angle xcex8.
FIG. 2 is a perspective view showing a laser diode with a horizontally tapered spot size converter in accordance with another conventional method.
However, there is a limit in reducing defects during the regrowth in the butt-joint portion BJ, that is, a (011) or (0-1-1) plane where the MQW active layer is exposed. In particular, it is very hard to control the growth condition, because the SSC should be regrown in a plane of the butt-joint portion whose crystal plane is not exposed clearly.
There is an advantage of no use of regrowth technique for spot size converter. However, the fine etching technique is necessary to form the very sharp tip for narrower radiation angle. It is very difficult to reproduce the horizontally tapered spot size converter
It is, therefore, an object of the present invention to provide a method for fabricating an optical device equipped with a spot size converter which can effectively lower reflectivity on the interface between the active layer and the spot size converter, and prevent defect from occurring during the regrowth.
In accordance with an embodiment of the present invention, there is provided a method for fabricating an optical device integrated with a spot size converter, comprising the steps of: a) depositing a first clad layer, an active layer and a second clad layer sequentially on the (100) plane of a semiconductor substrate; b) forming on the second clad layer a double dielectric mask of which the lower layer has a relatively wider width than that of the upper layer, exposing one side of the second clad layer; c) wet-etching the first clad layer, the active layer and the second clad layer in a buried ridge structure by using the double dielectric mask, and exposing the (111)A plane of the active layer tilted towards the (100) plane by a predetermined angle; d) growing a spot size conversion region on the (111)A plane of the active layer; and e) removing the double dielectric mask.